In Gratton et al. U.S. Pat. Nos. 5,497,769 and 5,492,118, an instrument based on multiple light sources, and using the principles of frequency domain spectroscopy, is described for the noninvasive determination of the light transmission properties of a scattering medium, such as absorption coefficient, reduced scattering coefficient, and index of refraction. By measuring the optical properties of living tissue, the instrument described in U.S. Pat. Nos. 5,497,769 and 5,492,118 can determine the concentrations of such materials as oxyhemoglobin, deoxyhemoglobin, glucose and the like. The principles of frequency spectroscopy are well known, being used in frequency domain fluorometry and/or phosphorimetry, being disclosed for example in Gratton U.S. Pat. No. 4,840,485 et al. and U.S. Pat. Nos. 5,212,386 and 5,323,010, among others.
Such probes typically operate using high frequency, sinusoidally modulated light. Typically, a probe is placed in contact with the surface of the medium to be measured. The average light intensity, the amplitude of the modulation in the light intensity, and the phase of the modulation are measured at multiple source detector separations, allowing the determination of the absorption coefficient, the scattering coefficient and/or the index of refraction of a highly scattering medium such as human or animal tissue. When sources of multiple wavelengths are employed, the concentrations of oxyhemoglobin and deoxyhemoglobin, for example, can be directly measured without the need for any a priori knowledge or estimation of the scattering or reflective properties of the tissue. This is an important advance, in that scattering within tissues varies widely from individual to individual, and among various tissues within an individual. Scattering may even change, over time, within a tissue.
However, the light sources and detector (or detectors) in these prior art techniques must be regularly calibrated with respect to a standard of known light transmission properties. This calibration must be repeated regularly since the intensity and/or phase of a light source may drift due to many factors such as time and temperature, and the sensitivity and phase response of the detector may also drift. Furthermore, this calibration cannot account for possible differences in light coupling efficiency between the standard of known properties and the sample to be measured. Hair, dirt, or the like may effect the efficiency with which the light is transmitted between the probe and the sample. Also, changes in the pressure used to hold the probe against the sample can affect the coupling efficiency of the light into the sample, as well as the coupling efficiency of the exit of the light from the sample to the detector.
In accordance with this invention, a new type of probe is provided, which probe has the advantage of allowing measurements similar to the above to be made without the need for calibration. The intensity of the light sources, the sensitivity of the light detectors, the high frequency phase response of the sources and detectors, the coupling efficiency of any fiber optic light guides present, and the coupling efficiency of light into and out of the highly scattering sample do not need to be known or precisely controlled. Also, correction factors do not need to be found by any calibration performed before or after the measurements, since the probes and the process described herein which are used to make measurements can be rendered independent of the above listed factors, by use of this invention. Thus, data acquired by this invention can be used to calculate the optical properties of samples and, by extension, the concentrations of various substances in the samples, including hemoglobin or glucose in living tissue.
The probes of this invention preferably do not contain moving parts, and are capable of obtaining desired data on a nearly instantaneous basis by irradiation of a highly scattering medium such as human or animal tissue, typically with light in the near infrared region between about 650 nm. and 1000 nm., where the light-absorbance of tissue is low. Such light may travel up to several centimeters through the tissue, providing a spectral window useful for photometric and spectrometric determination of tissue components.